None of the formats Lightroom supports will allow you to have a transparent background. The file would have to be converted to a GIF or PNG, and I don't know how you would be able to integrate that into the book module. It would probably be easier to create your own background layer using Photoshop or something similar, and creating the background color yourself. Then you could add your other image as a layer.

I actually work on a workaround and I think it is the same as you, JimHess, advise me to do but then I discovered another thing that I don't understand.

I thought that I should use the same color in the background of the pic as the color of my pages in my book. So I checked the color code in LR for the color of the pages and LR reports this settings;

N= 47, M=98%, L=81%. R=100%, G=92%, B=62% and HEX= FDEA9E

If I in PS set the NML to the same value it's not even close to the color as the pages in LR. If I set the RGB it's even worse but if I set the HEX in PS it is quite close but still a visible differens when I mount the pic in the book.

How can it be that the color difference so much between PS and LR? I thought that if you set the RGB, NML or the HEX it should be the exact same color.

Do I have to manually change the color in PS and then check it in LR and do this over and over again until I find the exact color as the pages have in my book?

What I want is that only the cut out part is shown in the book and the rest of the area of the picture frame is in the same color as the color of the pages (in my case light yellow).

Is it possible to do what I want to do and if it is, how do I do?

You should be able to do this by making the PSD file's Background layer the same color as LR's Book module Background color. The key to making this work properly is in matching the two Background colors. In LR's Book module click on the Background panels 'Background Color' color picker icon, right-click in the color picker area, and select HSB. In PS use Fill and set the color of the Background layer using the HSB values inside LR's Book module. Use LR's native ProPhoto RGB color space in PS to prevent the Background color from shifting inside LR.

I tried to do as you describe but I never succeed to get the correct background color of my photo compared to the page color in LR.

Finally I made a screen shot of one book page in LR, imported it to PS and used the color picker tool to find out the correct color values. Then I set that color as the background layer color and now I got a perfect match of the photos background color and the color of the book pages. I'm happy now.

Thank you all for the inputs so I started to think "outside of the box" and did find a solution that worked for me.

Try using your Hex color value FDEA9E in both LR and PS color pickers. Are you using ProPhoto RGB working space inside PS? Also make sure the PS background layer is a 'Background' layer and not a normal 'Layer' with Opacity set to less than 100%.

That was what I did first but there was a slight difference in the color, just visible for my eye, when I mount the photo in LR.

As an unexperienced user of PS I do not understand how to check if I use , I did try to find it out with PS Help but I did not find anything.

The HEX FEEAAF in PS have a perfect match (for my eyes) to HEX FDEA9E in LR, the difference might indicate that I'm not using ProPhoto RGB working space inside PS. How do I check this (or how do I open a new Background with this setting)?

The HEX FEEAAF in PS have a perfect match (for my eyes) to HEX FDEA9E in LR, the difference might indicate that I'm not using ProPhoto RGB working space inside PS. How do I check this (or how do I open a new Background with this setting)?

I get the same results when using an sRGB color profile image. Notice the LR background color at the Top and Bottom of the page below is slightly different than the image file. Both PS and LR Backgrounds were set to FEEAAF, so we have confirmed this is the issue!

When you open an image inside PS Using 'Edit in PS' from LR it uses your LR 'External Editing' Preferences settings. Set your LR's Preferences as below:

If you are using File> Open from inside PS you can check the file's color profile using Edit> Convert To Profile and select 'Destination Space' >RGB > ProPhoto RGB. In the image file used below it is already using ProPhoto RGB (i.e. Source Space).

Your PS Working Color Space should be set to ProPhoto RGB with warnings enabled using Edit> Color Settings as shown below from inside PS.